Scientists from St. Jude Children’s Research Hospital and the University of Texas Southwestern Medical Center have conducted a study on a transporter involved in cancer and immunity. By capturing six different structures of the transporter, including its binding to an inhibitor, the researchers have gained unprecedented insights into its structure and function. This breakthrough, published in the journal Cell, carries significant implications for the development of new drugs.
Transporters play a crucial role in facilitating the movement of substances across cell membranes, enabling them to carry out their functions. One such molecule, sphingosine-1-phosphate (S1P), is a vital signaling molecule involved in regulating various processes such as the immune system, blood vessel formation, auditory function, and the integrity of epithelial and endothelial membranes. Importantly, S1P also supports the growth and survival of cancer cells by promoting chemoresistance and metastasis.
While S1P is synthesized within the cell, it needs to cross the cell membrane to fulfill its signaling duties. Spinster homolog 2 (Spns2) is an S1P transporter protein that resides on the cell membrane. It opens inwardly, binding to S1P, and then opens outwardly to release it outside the cell.
Previous research has indicated that modulating the activity of Spns2 can have therapeutic benefits in treating cancer, inflammation, and immune-related disorders. However, the precise transport mechanism of Spns2 and how to inhibit it have remained unclear.
Co-corresponding author Dr. Chia-Hsueh Lee from St. Jude’s Department of Structural Biology expressed hope that their newfound structural knowledge would facilitate the development of more effective and specific small molecules with increased potency against Spns2 in the future. Dr. Lee believes that inhibiting the Spns2 transporter holds immense potential as a therapeutic approach.
Cryo-EM structures explain how the transporter works
In their study, the researchers utilized cryo-Electron Microscopy (cryo-EM) to obtain six distinct structures of Spns2, including two intermediate conformations that play a functional role in connecting the inward and outward states of the transporter. These findings offer valuable insights into the structural basis of the S1P transport cycle.
Dr. Lee expressed satisfaction with the results, noting that capturing the major conformations of a specific transporter is a rare achievement. By comparing and analyzing these different structures, the researchers gained a highly detailed understanding of how Spns2 interacts with the S1P signaling molecule.
Dr. Ahmed, one of the co-first authors from St. Jude’s Department of Structural Biology, explained that cryo-EM was instrumental in visualizing the transporter’s structure and elucidating how it moves S1P from inside to outside the cells. The team also investigated the binding of an inhibitor called 16d, a specific small molecule known for its minimal off-target effects. The structural data obtained shed light on how the inhibitor binds to the transporter, effectively blocking its activity.
The researchers discovered that 16d prevents transport activity by trapping Spns2 in an inward-facing conformation. This inhibition prevents the transition of the protein from the inward to outward state and hinders the transfer of the signaling molecule from inside to outside the cells. Furthermore, the inhibitor physically obstructs the binding of the signaling molecule by occupying the same cavity as its binding site.
Cell surface molecules present attractive targets for drug development, as demonstrated by the success of therapeutics targeting G-protein coupled receptors (GPCRs). Transporters, being cell surface molecules themselves, hold similar potential for drug development. Consequently, comprehending their structure and function is crucial for advancing disease treatments.
Dr. Li, the co-corresponding author from the Departments of Molecular Genetics and Biophysics at the University of Texas Southwestern Medical Center, highlighted the significance of their work in revealing the atomic details of the Spns2-mediated S1P transport cycle. This knowledge proves vital for understanding how this signaling sphingolipid circulates in the immune system. Additionally, the structures obtained contribute to the development of potent Spns2 inhibitors, which have the potential to enhance cancer and autoimmune disease treatments.